Pump Impeller Trim for a Balanced Flow

The affinity relations for a TRIM are not quite the ones for a speed change, even though they look the same: flow scales with diameter directly, head with the square and power with the cube, but the correspondence is approximate because trimming changes the impeller's geometry relative to its casing rather than scaling the whole machine. The manufacturer's published trim curves are the authority; these relations give a first estimate accurate enough to decide whether trimming is worth pursuing at all. The saving is real and continuous. A pump throttled to reduce flow is developing head the system does not need and then destroying it across a balance valve, and that head times that flow is power converted directly into water temperature. A trimmed impeller never develops the excess head in the first place, so the saving persists for the life of the pump with no control action and nothing to fall out of adjustment. Because power goes as the CUBE of the diameter ratio, a modest trim is a large power reduction -- which is what makes the machine-shop cost pay back in months rather than years. Two checks decide whether the trim is usable, and both are computed here rather than left as caveats. The first is head: the trimmed pump must still make the required HEAD at the required flow, not merely the flow, and a trim that delivers one without the other is not a solution. The second is the practical limit: below roughly three quarters of the casing's maximum diameter the gap between impeller tip and casing grows, the hydraulic match degrades, efficiency falls off, and the affinity estimate itself becomes unreliable -- at which point the manufacturer will usually recommend a different pump or a smaller casing. This is an estimate against published curves: it does not read a pump curve, compute efficiency at the trimmed condition, check NPSH available against the new requirement, or address the minimum flow the pump needs. The pump manufacturer's trim curves and the mechanical engineer of record govern.

trim affinity: Q2/Q1 = D2/D1, H2/H1 = (D2/D1)^2, P2/P1 = (D2/D1)^3; the required diameter is the current one times the flow ratio; the head at the trimmed diameter is checked against the head required; and the annual saving is the motor power times one minus the power ratio, over the operating hours.

The impeller-trim affinity relations as pump practice writes them, noting that a TRIM is not a speed change: it alters the impeller's geometry relative to its casing, so the correspondence is APPROXIMATE and the manufacturer's published trim curves are the authority. The practical limit of roughly 75 to 80% of the casing maximum is ENTERED. It does not read a pump curve, compute efficiency at the trimmed condition, check NPSH available, or address minimum flow.

Three affinity ratios and one energy calculation.

Estimate. AHJ and licensed professional govern.

Field names used by the API: current_diameter_in, current_flow_gpm, required_flow_gpm, current_head_ft, required_head_ft, max_diameter_in, min_trim_fraction, motor_hp, annual_hours, energy_rate_per_kwh, required_diameter_in, trim_in, trim_pct, power_ratio, head_at_trim_ft, annual_kwh_saved, annual_cost_saved

• The trim relations are approximate trimming changes geometry relative to the casing, unlike a speed changethe manufacturer's trim curves
• No efficiency at the trimmed point efficiency falls as the tip-to-casing gap growsthe pump manufacturer
• NPSH is not checked the new operating point has its own requirementthe mechanical engineer of record